1. Field of Endeavor
The present invention relates to the field of working on workpieces with water jets. It concerns a method for working on, in particular cleaning, a workpiece with a water jet that contains abrasive and emerges from a nozzle under high pressure, and to a water jet installation useful for executing the method. Moreover, the invention relates to a method for application of the water jet method.
2. Brief Description of the Related Art
Components of power plant installations are subject to high mechanical and thermal load during their operation. This applies particularly to gas turbine components exposed to the flow of hot gas, whose surfaces, in addition to being exposed to the extreme mechanical and thermal loads, are additionally exposed to unwanted thermal and chemical reactions with the formation of non-metallic layers, such as scale or corrosion coverings, with negative effects on the operating behavior. This necessitates regular service intervals for checking the state of these components and removing and/or cleaning, repairing or, if necessary, replacing them.
Methods for cleaning gas turbine components such as, for example, blades, are known in a multiplicity of realizations. The methods that are known and have been introduced in this field include that of sand blasting. Air that is compressed to a plurality of bars and to which an abrasive material is added, is directed on to the surface to be treated. The particles of the abrasive material impacting with high energy on the surface produce a cleaning effect. Disadvantages of this method, however, are an imprecise scattering and a relatively coarse removal of material, with disadvantageous alterations of the surface quality of the workpiece.
Another type of cleaning method is based on the high-pressure water jet technique, wherein pure water jets or water jets mixed with an abrasive are applied to the surface to be cleaned. The high-pressure water jet technique uses water pressures of up to 600 MPa, in order to produce a high-power water jet. Such a high-power water jet can be used as a tool for cutting or cleaning applications that acts in all directions.
Depending on the respective application, water jets operating according to three differing principles are used, namely:
(1) pure water jets (see FIG. 1),
(2) water jets containing abrasive that are generated through injection of an abrasive into a previously produced pure water jet (abrasive injection water jets, AIWJ; see FIG. 2), and
(3) water jets containing abrasive in which the jet is produced through a pressurized suspension of the abrasive emerging from a nozzle (abrasive suspension water jets, ASWJ; see FIG. 3).
In the case of the first principle, represented in simplified form in FIG. 1, in a water jet installation 10 water is fed to a pressure pump 12 via a water feed line 11 and pumped at high pressure into a pressure line 13, which leads to a suitable nozzle 14. The water under high pressure in the pressure line 13 then emerges from the nozzle 14 as required, forming a high-energy water jet. Such pure water jets can be used to cut soft materials such as, for example, fabrics, leather, solidified foams, foodstuffs, etc.
For cleaning applications, it is mainly systems operating with a pure water jet that are used. Typical parameters for cleaning with a pure water jet are working pressures of up to 300 MPa and volumetric flow rates of approximately 30 liters/min, which result in a high energy consumption (up to 150 kW). Corresponding high-pressure pumps are likewise very expensive.
In the case of the second principle, represented in FIG. 2, in a water jet installation 20 water is again fed to a pressure pump 12 via a water feed line 11 and pumped at high pressure into a pressure line 13, which leads to a suitable nozzle 14. The water under high pressure in the pressure line 13 then emerges from the nozzle 14 as required, forming a high-energy water jet. In a succeeding mixing tube 16, an abrasive is then admixed to the pure water jet, in an injection device 17, which abrasive has been brought via an abrasive feed 18. A high-energy water jet 19 containing abrasive then emerges at the end of the mixing tube 16. Such an installation is described, for example, in the printed publication WO-A1-2005/051598. Such AIWJ jets (abrasive injection water jets) are used mainly in stationary cutting applications. They can be used to cut all technical materials, such as:
all metals (steel, aluminum, copper, titanium, etc)
glass
synthetic materials
composite materials, and
concrete.
The ASWJ jets (abrasive suspension water jets) produced according to the third principle are generally used for mobile and special applications. The advantages of the ASWJ jets, as compared with the AIWJ jets produced according to the second principle, are a higher efficiency (higher by a factor of up to 4-5) and the possibility of being able to use these jets in all positions and environments.
In the case of the third principle, represented in FIG. 3, in a water jet installation 30 water is again fed to a pressure pump 12 via a water feed line 11 and pumped at high pressure (up to 200 MPa) into a pressure line 13, which leads to a suitable nozzle 14. At a T-piece 21, the flow of water is divided. One portion flows directly to the nozzle 14, via a first choke valve 27 and a mixing piece 28. A second, smaller portion flows in a bypass line 23, via a second choke valve 22, into a pressure tank 24 that is filled with abrasive and that is refillable after removal of a blind plug 25, and from there flows, via a shutoff valve 26, to the mixing piece 28. As the water flows through the pressure tank 24, it carries the abrasive particles along with it. Then, in the mixing piece 28, the resultant water/abrasive mixture is put into the main water flow. The proportion of abrasive in the water jet 29 that contains abrasive and emerges from the nozzle 14 can be controlled by the choke valves 22 and 27. Such a system is described, for example, in the printed publication DE-A1-199 09 377.
The main disadvantages of the currently known systems operating, according to the third principle, with pressures of between 50 MPa and 200 MPa are:
the imprecise control of the proportion of abrasive in the suspension;
the lack of possibility of continuous operation, since after a certain period of time it is necessary to interrupt operation and refill the pressure tank with abrasive; and
the high working pressures require correspondingly dimensioned components of the water jet installation, with the consequence of more difficult handling and a limited scope of application in respect of confined spatial conditions.